In motor vehicles, FMCW radar sensors are used to detect the traffic environment, in particular to localize other vehicles. The localization results can be used for a variety of assistance functions, for example for automatic separation control, automatic collision warning, or also automatic triggering of an emergency braking procedure in the case of an acute risk of collision.
In frequency modulated continuous wave (FMCW) radar sensors, a transmitted signal whose transmission frequency is modulated in ramp-shaped fashion is used, the signal being transmitted continuously during the course of the ramp. A baseband signal is generated from a received signal by mixing with the transmitted signal, and is sampled and evaluated.
The frequency of the baseband signal corresponds to the frequency difference between the signal transmitted at a given point in time and the signal received at the same point in time. Because of the frequency modulation of the transmitted signal, this frequency difference depends on the transit time of the signal from the radar sensor to the object and back, and thus on the distance of the object. Because of the Doppler effect, however, the frequency difference also contains a component that is conditioned by the relative velocity of the object. A measurement of the frequency difference on a single ramp therefore does not yet permit a determination of the distance and the relative velocity, but instead supplies only a linear relationship between those variables. This relationship can be depicted as a straight line on a distance-velocity diagram (d-v diagram).
There are conventional FMCW radar sensors that work with a sequence of identical, comparatively short ramps, called “rapid chirps,” which have a large frequency swing in relation to their duration and are therefore so steep that the distance-dependent component of the frequency shift dominates in the baseband signal while the Doppler shift is sampled by the sequence of ramps. A sufficiently high repetition rate of the short ramps is therefore necessary in order to arrive at an unambiguous determination of the relative velocity within a desired measurement region of the relative velocity. In particular, the time offset between successive short ramps must be less than half the period length of the Doppler frequency.
In order to enable an accurate estimate of the velocity and distance of radar objects with a minimal hardware outlay and computation outlay, it has been proposed to use several successive sequences of frequency modulation ramps with a temporal spacing of the ramps, in which an undersampling of the Doppler shift occurs over the sequence of ramps so that the information obtained regarding the relative velocity is affected by an ambiguity. By selecting different ramp parameters for the sequences, relative velocity values having different ambiguities and different linear correlations between the distance and velocity can be ascertained from the different sequences. A method of this kind, in which two sequences of short ramps having a different ramp repetition time are transmitted in succession, is described, for example, in German Patent Application No. DE 10 2012 212 888 A1. A value for the relative velocity of an object is determined by equalizing the information obtained for the different sequences.